In many jurisdictions and situations, incumbents have structural advantages over challengers during elections. The timing of elections may be determined by the incumbent instead of a set schedule. For most political offices, the incumbent often has greater name recognition due to their previous work in the office. Incumbents also have easier access to campaign finance, as well as government resources (such as the franking privilege) that can be indirectly used to boost a campaign. An election (especially for a legislature) in which no incumbent is running is often called an open seat; because of the lack of incumbency advantage, these are often amongst the most hotly-contested races in any election.

Incumbency is an especially powerful force within political parties. In the United States, incumbents traditionally win their party's nomination to run for office. Unseating an incumbent president, senator or other figure during a primary election is very difficult. In particular, barring major scandal or controversy, about 90% of congressional incumbents win re-election to their seats. However, shifts in congressional districts due to reapportionment or other longer-term factors may make it more or less likely for an incumbent to win re-election over time. For example, a Democratic incumbent in historically conservativeTexas would have a less likely chance of winning than a Democratic incumbent in liberalNew York City, because Texas has shifted away from the Democratic party in terms of voting.

In countries such as Canada, where nomination systems are also used, it is also difficult to remove an incumbent within the party.

However, there exist scenarios in which the incumbency factor itself leads to the downfall of the incumbent. Popularly known as the anti-incumbency factor, situations of this kind occur when the incumbent has proven himself not worthy of office during his tenure and the challenger demonstrates this fact to the voters. An anti-incumbency factor can also be responsible for bringing down incumbents who have been in office for many successive terms in spite of performance indicators, simply because the voters are convinced by the challenger of a need for change.

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99 percent of Senate and 90 percent of House incumbents were reelected.

When newcomers vie to fill an open office, voters tend to compare and contrast the candidates' qualifications, issues positions and personal characteristics in a relatively straightforward way. Elections featuring an incumbent, on the other hand, are as Molyneux puts it, "fundamentally a referendum on the incumbent." Voters will first grapple with the record of the incumbent. Only if they decide to "fire" the incumbent do they begin to evaluate whether the challenger is an acceptable alternative.

Voters typically know incumbents well and have strong opinions about their performance. Challengers are less familiar and invariably fall short on straightforward comparisons of experience and (in the presidential arena) command of foreign policy. Some voters find themselves conflicted -- dissatisfied with the incumbent yet also wary of the challenger -- and may carry that uncertainty through the final days of the campaign and sometimes right into the voting booth. Among the perpetually conflicted, the attitudes about the incumbent are usually more predictive of these conflicted voters' final decision than their lingering doubts about the challenger. Thus, in the campaign's last hours, we tend to see "undecided" voters "break" for the challenger.